Anisotropic Fiber Research Articles

Role of infarct scar dimensions, border zone repolarization properties and anisotropy in the origin and maintenance of cardiac reentry

Cardiac ventricular tachycardia (VT) is a life-threatening arrhythmia consisting of a well organized structure of reentrant electrical excitation pathways. Understanding the generation and maintenance of the reentrant mechanisms, which lead to the onset of VT induced by premature beats in presence of infarct scar, is one of the most important issues in current electrocardiology. We investigate, by means of numerical simulations, the role of infarct scar dimension, repolarization properties and anisotropic fiber structure of scar tissue border zone (BZ) in the genesis of VT. The simulations are based on the Bidomain model, a reaction-diffusion system of Partial Differential Equations, discretized by finite elements in space and implicit-explicit finite differences in time. The computational domain adopted is an idealized left ventricle affected by an infarct scar extending transmurally. We consider two different scenarios: i) the scar region extends along the entire transmural wall thickness, from endocardium to epicardium, with the exception of a BZ region shaped as a central sub-epicardial channel (CBZ); ii) the scar region extends transmurally along the ventricular wall, from endocardium to a sub-epicardial surface, and is surrounded by a BZ region (EBZ). In CBZ simulations, the results have shown that: i) the scar extent is a crucial element for the genesis of reentry; ii) the repolarization properties of the CBZ, in particular the reduction of IKs and IKr currents, play an important role in the genesis of reentrant VT. In EBZ simulations, since the possible reentrant pathway is not assigned a-priori, we investigate in depth where the entry and exit sites of the cycle of reentry are located and how the functional channel of reentry develops. The results have shown that: i) the interplay between the epicardial anisotropic fiber structure and the EBZ shape strongly affects the propensity that an endocardial premature stimulus generates a cycle of reentry; ii) reentrant pathways always develop along the epicardial fiber direction; iii) very thin EBZs rather than thick EBZs facilitate the onset of cycles of reentry; iv) the sustainability of cycles of reentry depends on the endocardial stimulation site and on the interplay between the epicardial breakthrough site, local fiber direction and BZ rim.

Microstructural abnormalities in callosal fibers and their relationship with cognitive function in schizophrenia: A tract-specific analysis study.

IntroductionThe corpus callosum serves the essential role of relaying cognitive information between the homologous regions in the left and the right hemispheres of the brain. Cognitive impairment is a core dysfunction of schizophrenia, but much of its pathophysiology is unknown. The aim of this study was to elucidate the association between microstructural abnormalities of the corpus callosum and cognitive dysfunction in schizophrenia.MethodsWe examined stepwise multiple regression analysis to investigate the relationship of the fractional anisotropy (FA) of callosal fibers in each segment with z‐scores of each brief assessment of cognition in schizophrenia subtest and cognitive composite score in all subjects (19 patients with schizophrenia [SZ group] and 19 healthy controls [HC group]). Callosal fibers were separated into seven segments based on their cortical projection using tract‐specific analysis of diffusion tensor imaging.ResultsThe FA of callosal fibers in the temporal segment was significantly associated with z‐scores of token motor test, Tower of London test, and the composite score. In the SZ group, the FA of callosal fibers in the temporal segment was significantly associated with the z‐score of the Tower of London test. In addition, the FA of callosal fibers in temporal segment showed significant negative association with the positive and negative syndrome scale negative score in the SZ group. Compared to the HC group, the FA in temporal segment was significantly decreased in the SZ group.ConclusionOur results suggest that microstructural abnormalities in the callosal white matter fibers connecting bilateral temporal lobe cortices contribute to poor executive function and severe negative symptom in patients with schizophrenia.

Effect of Continuous Micro Reinforcement and Processing Parameters on the Low-Velocity Impact Behaviour of Polymer Composite Materials

Low velocity impact tests were conducted on quasi-isotropic [�45/0/90o]xs laminates under drop weight impact from 0.7m, corresponding to a 30J energy. In this respect modified epoxy blends reinforced with carbon and Kevlar woven fabrics laminates were developed using autoclave technology. The four configurations developed for low velocity impact tests aimed at investigating several aspects like: the effect of fiber type, stacking sequence and mainly technological processing parameters, on the impact performances. The recorded Load-Time curves were plotted and visual inspection, high resolution laser scanner were used to observe the fracture characteristics of the impacted composite laminates. The results obtained showed that for tested configurations, both stacking sequence and processing parameters directly linked to fiber volume fraction, have a strong effect on the impact performances. The amount of absorbed energy, ductility index was calculated for each configuration under study. The results obtained showed that hybrid configuration exhibits lower stiffness and damage initiation energy amount when compared to carbon reinforced configurations. Nevertheless, their damage propagation energy amount and ductility index was the uppermost. This behaviour was already reported previously [1] and is partially attributed to the higher elastic energy absorption of carbon fibers that delays the propagation of delamination, and fiber breakage. Lower tenacity obtained on hybrid laminates was attributed to both lack of resin local rinse saturate and to the intrinsic anisotropy of para-aramid fibers.

Microscopic Measurement of Strain Fields with Digital Image Correlation and Comparison to Finite Element Modelling

Digital Image Correlation (DIC) has become more and more important in the field of material characterization and research, especially for strongly anisotropic fiber reinforced materials. Its big advantage over the conventional methods like strain gauges or point based video-extensometers is the full field strain and displacement measurement and the ability to analyze three-dimensional displacements. Although theoretically, the concept of the DIC as a pure image-based method allows it to work on every imaginable scale, its main field of application is in the range, where the region of interest (ROI) has a size between 10 −2 m to 10 −1 m. In this case, imaging is accomplished with the use of high-resolution black and white digital cameras. This work is focused on a smaller scale with ROI sizes between 10 −4 m to 10 −3 m, where a digital microscope is used to create the images. The innovative idea behind this work is using the natural surface structure of a polished carbon fiber reinforced Polyamide-6 sample, produced by automated fiber placement, as a statistical pattern instead of the usual speckle pattern applied to the area to be investigated. This way the stress and strain distributionin different regions of the investigated sample area can be evaluated and displayed, while the sample is exposed to an increasing mechanical load in form of a three-point bending test. The resulting strain and displacement fields are compared to finite element modeling of the ROI. To provide an accurate model, the image of the sample is first segmented into fiber, matrix and voids using “Trainable Weka Segmentation” and the resulting phases mapped with the corresponding material properties. To compute the resulting strains in the sample, the measured displacements from the DIC on the edges of the ROI were used as boundary conditions for the simulation. Simulation and experimental results clearly point out the inhomogeneity of the strain field in these samples. Due to the presence of fiber rovings and the presence of voids, local strain values exceed the global average by up to 4 %.

Efficient Characterization and Modelling of Material Behaviour of LFT for Component Simulations

Modeling failure and progressive damage of long fibre reinforced thermoplastics (LFT) presents a challenging task since local inhomogeneities, anisotropic fibre orientations, and strain-rate dependence must be taken into account also on the microscale. We show that for simple geometries, the material behaviour of the composite can be modelled using layered geometrical models. But for more complex geometries, this approach fails since the fibre orientation distribution is inhomogeneous. In this case, multiscale methods allow the accurate and efficient prediction of the material behaviour with the local fibre orientation taken from an injection molding simulation. This material model can be extended to viscoplasticity and integrated into the NTFA-TSO method from Michel & Suquet (2016). In this way, we can obtain an accurate and efficient multiscale method for the realistic modelling of LFT.

Modelling of polarised optical frequency domain reflectometry of axially twisted anisotropic optical fibres

This paper presents a mathematical model that describes the influence of defects in an anisotropic optical fibre on its polarised frequency domain reflectograms. The model relies on the Jones matrix formalism. Calculated reflectograms are presented for a uniform axial twist of an optical fibre. As input data for simulation, we use real distributed optical parameters of the fibre determined by polarised light Brillouin reflectometry.

A long-range force based random method for generating anisotropic 2D fiber arrangement statistically equivalent to real composites

A new method to generate 2D fiber arrangements in a transverse representative volume element (RVE) of composites is presented in this paper. Inspired by molecular dynamics, this method introduces the concept of “intermolecular force”, which has a great influence on the distribution of generated arrangements. Fiber arrangements of real composites are obtained by optical microscopy and analyzed by statistical functions, and the results show that the distribution of fibers is anisotropic. Then, statistical functions are implemented to analyze the distribution of RVEs generated by the proposed method. The results match well with the experimental results in all statistical functions, proving the statistical equivalence of generated arrangements to real composites. Finally, the elastic properties of RVEs in different orientations are simulated by the finite element method (FEM). The results indicate that the transverse mechanical properties are anisotropic. This simple and highly efficient method can generate fiber arrangements with fiber volume fraction up to 80%. Besides, this method has enough parameters to control the fiber distribution and can generate anisotropic fiber arrangements statistically equivalent to the real composites easily.

Simultaneous fMRI-EEG-DTI recording of MMN in patients with schizophrenia.

Functional magnetic resonance imaging (fMRI), electroencephalogram (EEG), and diffusion tensor imaging (DTI) recording have complementary spatiotemporal resolution limitations but can be powerful methods when used together to enable both functional and anatomical modeling, with each neuroimaging procedure used to maximum advantage. We recorded EEGs during event-related fMRI followed by DTI in 15 healthy volunteers and 12 patients with schizophrenia using an omission mismatch negativity (MMN) paradigm. Blood oxygenation level-dependent (BOLD) signal changes were calculated in a region of interest (ROI) analysis, and fractional anisotropy (FA) in the white matter fibers related to each area was compared between groups using tract-specific analysis. Patients with schizophrenia had reduced BOLD activity in the left middle temporal gyrus, and BOLD activity in the right insula and right parahippocampal gyrus significantly correlated with positive symptoms on the Positive and Negative Syndrome Scale (PANSS) and hostility subscores. BOLD activation of Heschl’s gyri also correlated with the limbic system, including the insula. FA values in the left anterior cingulate cortex (ACC) significantly correlated with changes in the BOLD signal in the right superior temporal gyrus (STG), and FA values in the right ACC significantly correlated with PANSS scores. This is the first study to examine MMN using simultaneous fMRI, EEG, and DTI recording in patients with schizophrenia to investigate the potential implications of abnormalities in the ACC and limbic system, including the insula and parahippocampal gyrus, as well as the STG. Structural changes in the ACC during schizophrenia may represent part of the neural basis for the observed MMN deficits. The deficits seen in the feedback/feedforward connections between the prefrontal cortex and STG modulated by the ACC and insula may specifically contribute to impaired MMN generation and clinical manifestations.

Effect of Splitting of Polymer Fibers on the Strength of Unidirectional Composites

With the example of Aramid fibers, it is shown that the model of acumulation of fiber breaks in composites cannot explain their observed tensile strength exceeding that of unimpregnated Aramid fibers. The model of longitudinal splitting of fibers proposed by Wagner, with account of fiber-matrix interaction, is based on the idea that the break of fibers begins at great strains owing to their interaction with the matrix. This model better describes the failure of composites with anisotropic fibers having an internal fibrillar structure. The results obtained can also be used for other types of fibers with an internal microstructure.

Locating Defects in Anisotropic CFRP Plates Using ToF-Based Probability Matrix and Neural Networks

This paper presents two algorithms, both based on the time of flight (ToF) of scattered waves, to locate defect in an anisotropic woven-fabric carbon fiber reinforced polymer (CFRP) plate. The first algorithm uses a probabilistic approach by constructing a probability matrix. Each element of this matrix is associated with a location and represents the probability of existing a defect at the corresponding spatial coordinates of the plate. For the probability matrix method, localization results are influenced by manually chosen parameters and by the anisotropy of the CFRP plate. The second algorithm, based on artificial neural networks (ANNs), enables us to improve the accuracy of locating defects. With this ANN method, the anisotropic feature can be “learned” by the neural network from training data. The ToF of scattered waves obtained from three sensor pairs were used directly as inputs of the neural network. The spatial coordinates of the defect are the ANN outputs. The difficulty of obtaining sufficient experimental data for ANN training was surpassed by using added blocks on the surface of the plate under test to simulate defects. This scheme was validated and proved to be effective by comparing the scattered waves from a delamination and from an added block. Conclusions have been drawn by comparing the two methods. The localization results obtained by the ANN algorithm are proved to be better.

Macrophage-Driven Biomaterial Degradation Depends on Scaffold Microarchitecture.

In situ tissue engineering is a technology in which non-cellular biomaterial scaffolds are implanted in order to induce local regeneration of replaced or damaged tissues. Degradable synthetic electrospun scaffolds are a versatile and promising class of biomaterials for various in situ tissue engineering applications, such as cardiovascular replacements. Functional in situ tissue regeneration depends on the balance between endogenous neo-tissue formation and scaffold degradation. Both these processes are driven by macrophages. Upon invasion into a scaffold, macrophages secrete reactive oxygen species (ROS) and hydrolytic enzymes, contributing to oxidative and enzymatic biomaterial degradation, respectively. This study aims to elucidate the effect of scaffold microarchitecture, i.e., μm-range fiber diameter and fiber alignment, on early macrophage-driven scaffold degradation. Electrospun poly-ε-caprolactone-bisurea (PCL-BU) scaffolds with either 2 or 6 μm (Ø) isotropic or anisotropic fibers were seeded with THP-1 derived human macrophages and cultured in vitro for 4 or 8 days. Our results revealed that macroph age-induced oxidative degradation in particular was dependent on scaffold microarchitecture, with the highest level of ROS-induced lipid peroxidation, NADPH oxidase gene expression and degradation in the 6 μm Ø anisotropic group. Whereas, biochemically polarized macrophages demonstrated a phenotype-specific degradative potential, the observed differences in macrophage degradative potential instigated by the scaffold microarchitecture could not be attributed to either distinct M1 or M2 polarization. This suggests that the scaffold microarchitecture uniquely affects macrophage-driven degradation. These findings emphasize the importance of considering the scaffold microarchitecture in the design of scaffolds for in situ tissue engineering applications and the tailoring of degradation kinetics thereof.

Heterogeneity, suspension, and yielding in sparse microfibrous cellulose gels 2: strain rate-dependent two-fluid behavior

Complex fluids are widely used in formulated products to impart rheological properties like stability, performance, and aesthetics. Yield stress fluids are a particularly important example, enabling applications like particle suspension, surface coating, and therapeutic delivery. Recent work has shown that particle suspension in yield stress fluids can be a strong function of yielding and flow heterogeneities, especially in anisotropic fiber dispersions. This work uses a microbubble technique to study the deformation before and during yielding of microfibrous cellulose suspensions. We note significant variations in suspension performance as a result of fiber rearrangement and heterogeneities. Strong strain rate dependencies are found to vary local network yield strength, and confocal microscopy quantifies structural reinforcement and deformation rate effects. The observed behavior indicates a two-fluid interpretation may help interpret sparse network flow and suspension properties.

Effect of the cutting speed on the cutting mechanism in machining CFRP

Cutting mechanism of CFRP is different from that of a homogeneous metal. For metal, high speed cutting can improve machining quality and efficiency. However, there is no agreed conclusion on the effect of cutting speed on the cutting mechanism of CFRP. This paper studies the effect of cutting speed on cutting mechanism of CFRP and damage formation by using finite element and experiment methods. A three-dimensional finite element model of orthogonal cutting CFRP is established considering the strain-rate-dependent mechanical properties of resin and the anisotropy of fiber. By this model, cutting processes at micro level are simulated. Fracture evolution processes of fiber and resin are characterized. Furthermore, experimental data of cutting forces are obtained, and the machined surface microstructure are observed based on orthogonal cutting experiments. Subsurface morphology and degree of resin cracking extension in simulation and experiment are consistent with each other. Results show that with the increase of cutting speed, the fiber deformation and the affected area before fiber fracture obviously deceases. Accordingly, the resin cracking and extension degree caused by the excessive deformation of fiber also be greatly reduced. In addition, there is a trend that the specific energy gap for different chip formation modes becomes smaller and smaller.

Strength anisotropy of fibre-reinforced sands under multiaxial loading

The strength anisotropy of fibre-reinforced sands in the multiaxial stress space has been investigated using a hollow cylinder torsional apparatus. Probing stress paths under constant cell pressure have been performed on both unreinforced and reinforced sand specimens to assess the influence of the orientation of the principal stress directions on the fibre strengthening contribution. For the first time, a deviatoric strength envelope for fibre-reinforced soils in the multiaxial stress space was identified. The addition of fibres produces an anisotropic increase and a distortion of the deviatoric strength envelope if compared to the unreinforced soil matrix. The fibre strengthening contribution is governed by the tensile strain domain developed and the fibre orientation distribution. Further observations on the effect of the addition of fibres on the volumetric response, principal stress and strain rate non-coaxiality, as well as the shear bands formation of the composite, are presented. An analytical model to capture the anisotropic fibre strengthening contribution is developed and discussed.

Symmetries and conservation laws of the Yao\u2013Zeng two-component short-pulse equation

This paper uses the Lie group method to analyze the symmetries of the Yao–Zeng two-component short-pulse equation which describes the propagation of polarized ultrashort light pulses in cubically nonlinear anisotropic optical fibers. Similarity reductions and exact solutions are obtained by constructing an optimal system of one-dimensional subalgebras. Moreover, the explicit solutions are constructed by the power series method and the convergence of power series solutions is proved. In addition, nonlinear self-adjointness and conservation laws of this system are discussed.

The role of long-period stacking ordered phases in the deformation behavior of a strong textured Mg-Zn-Gd-Y-Zr alloy sheet processed by hot extrusion

In this work, deformation behavior of an extruded Mg-Zn-Gd-Y-Zr alloy sheet, including mechanical anisotropy and tension-compression asymmetry, were investigated by conducting tension and compression experiments along different directions. Coupling with the strong texture, the role of long-period stacking ordered (LPSO) phases in the deformation behavior was analyzed based on the microscopic plastic deformation theory. The results showed that LPSO phases with different morphologies played different roles in deformation behavior of the strong textured Mg-Zn-Gd-Y-Zr alloy sheet: (i) the irregular-shaped LPSO phases distributed along extrusion direction (ED) brought anisotropic fiber reinforcement and exacerbated the mechanical anisotropy; (ii) the needle-like LPSO phases filled in the grain interior hindered the propagation of twinning boundaries and decreased the tension-compression asymmetry. The results may provide valuable references for the manufacturing of large size Mg/LPSO alloys with low mechanical anisotropy and tension-compression asymmetry, hence making them better to meet the growing need of engineering application.

Extrusion of cement-based materials - an overview

Extrusion is a process that consists in forcing a formable material to pass through a die having the cross-section of the part to be obtained. This way of processing is used with conventional and fibre-reinforced cement-based materials to fabricate various construction elements such as panels, pipes and roadside curbs. Recently, with the development of digital fabrication methods and especially 3D concrete printing by selective deposition, the extrusion techniques have experienced a significant increase in interest.
 This letter describes the screw and ram extrusion techniques and their applications in construction industry. Furthermore, the underlying mechanisms involved during extrusion flow are delineated and the roles of rheological and hydro-mechanical behaviours (the latter one in a soil mechanics sense) in defining the extrudability – ability of being extruded – of the cementitious materials are highlighted. Finally, specific points such as flow-induced anisotropy of fibre reinforced cementitious materials or surface defects are addressed.

Linear elliptic equations in composite media with anisotropic fibres

Linear elliptic equations in composite media with anisotropic fibres are concerned. The media consist of a periodic set of anisotropic fibres with low conductivity, included in a connected matrix with high conductivity. Inside the anisotropic fibres, the conductivity in the longitudinal direction is relatively high compared with that in the transverse directions. The coefficients of the elliptic equations depend on the conductivity. This work is to derive the Hölder and the gradient Lp estimates (uniformly in the period size of the set of anisotropic fibres as well as in the conductivity ratio of the fibres in the transverse directions to the connected matrix) for the solutions of the elliptic equations. Furthermore, it is shown that, inside the fibres, the solutions have higher regularity along the fibres than in the transverse directions.

Understanding the role of air and protein phase on mechanical anisotropy of calcium caseinate fibers

Calcium caseinate dispersions can be transformed into anisotropic, fibrous materials using the concept of shear-induced structuring. The aim of this study is to further investigate the relative importance of air bubbles and protein on the mechanical anisotropy of calcium caseinate material. In this study, the effect of air on mechanical anisotropy of these fibrous materials was described with a load-bearing model, with the void fraction, and the bubble length and width as input parameters. The anisotropy of the protein phase was estimated using materials obtained from deaerated dispersions after shearing at different shear rates. We concluded that the deformation of air bubbles can only partly explain the mechanical anisotropy; the anisotropy of the protein phase is more important. Based on all results, we further concluded that the anisotropy of the protein phase was affected by the air bubbles present during the structuring process. This effect was explained by locally higher shear rate in the protein matrix during the structuring process.

Analysis of Water Transport in Anisotropic Gas Diffusion Layers for Improved Flooding Performance of PEFC

To improve the flooding performance of a polymer electrolyte fuel cell (PEFC), the effect of fiber direction of gas diffusion layer (GDL) is investigated by a lattice Boltzmann method (LBM). Because much water is generally accumulated in the GDL under the ribs, transport of the water into the channel is the focus in this report. The results show that the anisotropic GDL with the fibers approximately distributed in the direction perpendicular to the rib reduces the water accumulated in the GDL compared with an isotropic GDL, but the anisotropic GDL with the strongly aligned fibers increases the water accumulation. This is because the water transport in the direction perpendicular to the rib is improved due to the flow along the anisotropic fibers, but the water transport characteristic in the thickness direction deteriorates due to the sharp and narrow pores in case of the strongly aligned fibers. The experiment developed in our previous study demonstrates that the anisotropic GDL with somewhat oriented fibers reduces the water in the GDL. Next, an attempt to improve the water transport is carried out. By the surface treatment reducing the contact angle of the GDL surface, the water accumulated in the anisotropic GDL is reduced.